Conventionally, various methods of detecting knocking (knock) of an internal combustion engine have been known. By way of example, a technique has been known which determines that knock has occurred when vibration intensity of an internal combustion engine is above a threshold value. It is possible, however, that noise such as vibration experienced when an intake valve or an exhaust valve is closed has intensity higher than the threshold value, while knocking does not occur. This may leads to an erroneous determination that knock has occurred, though knock has not occurred. Therefore, techniques for determining presence/absence of knocking based on vibration waveform to take into consideration characteristics other than the intensity, such as crank angle at which vibration occurs or attenuation factor have been proposed.
Japanese Patent Laying-Open No. 2003-021032 discloses a knock control device for an internal combustion engine, including a knock sensor for detecting knocking of the internal combustion engine, a statistical processing unit for statistically processing an output signal detected by the knock sensor, a first temporary determining unit for determining knock occurrence based on the result of processing by the statistical processing unit, a second temporary determining unit for determining knock occurrence based on the waveform of the output signal detected by the knock sensor, and a final knock determining unit for finally determining knock occurrence based on the temporary knock determination by the first temporary determining unit and the temporary knock determination by the second temporary determining unit. The final knock determining unit finally determines that knock has occurred, if both the first and second temporary determining units determined that knock has occurred. In the first temporary determining unit, whether knock has occurred or not is determined by comparing the maximum value of the output signal detected by the knock sensor with a knock determination level (determination value) calculated based on the result of processing by the statistical processing unit. Based on the frequency of knock occurrence, the determination value is corrected to a value obtained by subtracting a set value ΔV from the determination value, or to a value obtained by adding a product of a value A larger than “1” and the set value ΔV to the determination value.
Now, as in the knock control device described in Japanese Patent Laying-Open No. 2003-021032, if vibration caused by knocking and noise are used not distinguished from each other in correcting the determination value used for determining presence/absence of knocking, the determination value would possibly be corrected inappropriately, as the noise intensity is comparable to the vibration intensity caused by knocking. Correction of the determination value with the noise removed may be possible. If the determination value were corrected with noise removed, however, data necessary for correcting the determination value would be insufficient. This possibly leads to delayed correction of the determination value.